Herd immunity followed widespread use of the Haemophilus influenzae type b (Hib) conjugate vaccine, and the near-elimination of Hib led to speculation that other Haemophilus influenzae (Hi) types may emerge as causes of meningitis. For example, in Brazil, the incidence of Hib meningitis decreased by 69 percent during the first year after initiation of Hib conjugate immunization while the incidence of Hia meningitis increased 8-fold. The Netherlands Reference Laboratory reported that type a was observed only in children younger than 4 years old. Properties of Hia and Hib CP. Hi may be classified into three virulence groups of two, related to their CP structures. Types a and b, which are the most virulent, are composed of a neutral sugar, an alcohol (ribitol), and a phosphodiester;types c and f are composed of an N-acetylated aminosugar, a monosaccharide, and a phosphodiester;and (3) types d and e have a repeat unit of an N-acetylglucosamine and N-acetylmannosamineuronic acid. Compared with the other four types, Hib and Hia are resistant to the bactericidal effects of complement alone. Challenge of infant rats showed that the 50 percent infective dose (ID50) for bacteremia of both Hib and Hia was several logs lower than of the other types. Intranasal challenge of infant rats with type b or type a resulted in 55 to 90 percent bacteremia with type b and 35 percent with type a. The other types were not invasive. Invasive diseases caused by Hia. Rates of Hia disease have remained constant in the United States despite Hib vaccination. From 1998 to 2002, the Emerging Infections Program of the CDC conducted active laboratory- and population-based surveillance for Hi disease in data from nine sites with approximately 35 million people. Seventeen of 1,743 invasive isolates were Hia, which is an important cause of meningitis in certain populations such as White Mountain Apache Indian childrenwith an annual incidence of 254 cases/100,000 children of Hia meningitis. Hammitt et al. (Pediatr Infect Dis J 2005;24:453) reported an outbreak of invasive Hia disease among Native Alaska infants. During a 6-month period in two proximate villages, five Hia cases were documented. In Brazil, an incidence of Hia meningitis of 0.16 cases/100,000 person-years was reported. South Africa and native populations in Manitoba Canada also report Hia meningitis in children. Hia vaccine. The number of Hia cases is too low for a randomized, double-blinded, controlled trial. Yet, precedent exists for adding types within a species to a vaccine without evidence for efficacy. For example, the licensing of several pneumococcal types, including meningococcal groups Y and W135 and poliovirus type 2, was based on their safety and immunogenicity. The structural, experimental, and clinical properties of Hia CP closely resemble those of type b, and the increasing number of reports of Hia-invasive disease suggests that development of an Hia conjugate is warranted. Methods for conjugating type b CP to a protein are applicable to Hia. D-1,5-ribitolphosphate is a constituent of the CPs of Hia and Hib. We reported that the cell wall polysaccharide (PS) of B. pumilus Sh18 contains a poly-1,5-ribitolphosphate as a major component and that antibodies induced in mice by such conjugate cross-reacted with both Hia and Hib. We synthesized polyribitolphosphate chains containing either 8 or 12 repeat units, with the terminal keto groups used for conjugation to aminooxylated bovine serum albumin (BSA or tetanus toxoid. We injected the conjugates into mice, three times at 2-week intervals at 2.5 g/mouse and obtained sera a week later. ELISA demonstrated antibodies to both Hia and Hib, with the octamer conjugate a better immunogen than the dodecamer conjugate. Some of the tested sera showed bactericidal activity against both type a and type b as roughly correlated with their ELISA values. Other constructs and formulations are being studied. Clinical lots are expected to be studied for safety and immunogenicity in adults and children. There are too few Hia cases for a randomized, double-blinded, controlled trial;yet, precedent exists for adding types within a species to a vaccine without evidence for efficacy. For example, the licensing of several pneumococcal types, meningococcal groups Y and W135 and poliovirus type 2, was based on their safety and immunogenicity.